In everything from blockbuster movies to the imaginations of children, Tyrannosaurus rex is a bloodthirsty predator. It's fast and agile, able to chase down a Jeep and recover from being bowled over by King Kong. If you stand in front of a full-size T. rex skeleton in a museum, your own instincts might support that idea. The same would be true if you found yourself face to face with any other member of its family, Tyrannosauridae. These dinosaurs all had armlike forelimbs, a bipedal gait and huge mouths full of startlingly jagged teeth.

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Most of the time, the Tyrannosauridae were the largest carnivores in their respective ecosystems. And aside from Hollywood drama, there's some fossil evidence to suggest that they deserve their reputation as aggressive predators. From the side, a tyrannosaurid's jaws look like saw blades made from long, daggerlike teeth. The teeth themselves are serrated, too, ideal for cutting their way through meat. According to analysis of skull fossils, Tyrannosaurus could bite down with a force of 183,000 to 325,000 newtons (3,822 to 6,788 pounds per square foot) [source: Meers]. Other researchers used the damage from a T. rex bite on a Triceratops fossil to estimate that the tyrant lizard's bite exerted a force of about 3,000 pounds (143,641 newtons), similar to the bite force of an alligator [source: Leutwyler]. Either way, T. rex's bite would be an ample weapon against large, prehistoric prey.

­In contrast to its relatively small arms, T. rex had powerful legs. Its thigh bones were relatively long, a trait common in animals with good running endurance. This suggests that a tyrannosaur's legs were adapting for traveling over­ long distances for long periods of time, perhaps to chase down other dinosaurs.

Not all of the evidence for a predatory nature in T. rex comes from its own fossils. Many of the prey species that lived at the time have elaborate body parts that can be interpreted as defense mechanisms. Triceratops, for example, has a bony frill protecting its neck in addition to the three horns for which it is named. Though there were other predators against which the frill would be useful as defense, T. rex would have been the largest most of the time.

But how does this evidence stack up to the other side of the coin -- the idea that T. rex was basically a giant Cretaceous vulture? Would those powerful legs really allow a massive dinosaur to move quickly enough to capture prey? And is there any evidence that tyrannosaurids hunted? We'll explore these questions on the next page.

Cretaceous Vultures: Tyrannosaur Scavengers

Was Tyrannosaurus rex a predator, or was it a scavenger like this California condor (Gymnogyps californianus)?

The idea that T. rex might have been a scavenger has been around since the early 1900s. The most famous proponent of the theory today is John R. Horner of Montana State University. According to Horner and other paleontologists, there are lots of reasons why T. rex may not have been a predator.

­One argument plays off the theory that dinosaurs evolved into birds. Some of the world's largest flying birds, such as condors, are scavengers -- they eat what they find instead of what they kill. By this logic, really big dinosaurs might have been scavengers, just like their really big avian counterparts today.

This is mostly speculative, but some aspects of T. rex anatomy suggest that it was a scavenger. Its nasal passages, for instance, are huge, potentially perfect for smelling faraway carrion. A tyrannosaurus's teeth and jaw are made for biting -- hard. When a T. rex closed its mouth, the lower teeth met the inside of the upper teeth, concentrating lots of force upward from the inside and downward from the outside. This force could break a bone just like you could break a stick if you bend it with two hands. Paleontologists have also analyzed a coprolite, or a pile of fossilizedT. rex dung, and found bone fragments inside. This may mean that the dinosaur relied on picked-over bones for nourishment. To some, the presence of lots of broken teeth also suggests that T. rex chewed its way through bones out of necessity, damaging its teeth in the process.

The scavenger theory applies to a tyrannosaur's body, too. Calculations made by paleontologist James Farlow suggest that T. rex was so massive that it would have sustained life-threatening injuries if it fell while running [source: Hecht]. There's also the matter of T. rex's almost comically undersized forelimbs, which would not be of use in breaking a fall or helping the animal regain its footing.

Scientists also debate whether the dinosaur could run at all, and it would need to run to capture prey. There's no footprint, or trackway, evidence to suggest that it did, but most trackways aren't big enough to encompass the stride a T. rex would have used when running. Several studies analyzing different aspects of tyrannosaur physiology suggest that T. rex's speed topped out at about 10 meters per second, or 22 mph, but others suggest that it may have run much faster. Researchers John Hutchinson and Mariano Garcia suggest that tyrannosaur legs couldn't have supported enough muscle to allow fast running [source: Fitzgerald].

According to Graeme D. Ruxton and David C. Houston of the University of Glasgow, T. rex could have supported itself by eating only dead animals if the ecosystem had about the same amount of carrion as today's Serengeti. The two came to this conclusion by studying the energy it would take to find food compared to the energy the food would provide the dinosaur [source: Ruxton and Houston].

So it was possible that T. rex was a scavenger -- but was it likely? Next, we'll explore some of the most common counterarguments.

Cretaceous Hyenas: Tyrannosaur Opportunists

One possible discovery could offer proof that Tyrannosaurus rex was a scavenger -- evidence of healed-over tyrannosaur bites in other fossils. Most fossilized dinosaurs died from events unrelated to being eaten, so it should be easy to tell whether they survived past attacks from predators. Unfortunately, there are few fossils that show clear evidence of past T. rex bites.

John W. Happ of Shenandoah University suggests a Triceratops whose skull was discovered in 1997 lived for years after being bitten by a T. rex [source: Perkins]. An Edmontosaurus fossil shows evidence of vertebral spines that re-grew after a bite that may have been inflicted by a tyrannosaur [source: Carpenter]. But there are few such examples, and the cause of the bone damage is difficult to prove conclusively. There's also the question of whether any animal would survive long enough for its T. rex bite to heal.

Some of the same evidence used to suggest that T. rex may have been a predator can be interpreted differently. For example, T. rex had tiny arms, especially compared to the prehistoric raptors. But some researchers point out that armlike forelimbs aren't a prerequisite for being a predator -- snakes, for instance, manage without them. One study also defies the idea that these limbs were useless. Kenneth Carpenter of the Denver Museum of Natural History suggests that based on the size, shape and position of the arm bones and shoulder blades, these apparently puny limbs were powerful.

Then there's the matter of the abundance of broken dinosaur teeth some interpret as evidence of eating abandoned bones. If T. rex was a predator, it might have used one of many methods to attack prey, and each could have resulted in broken teeth. Tyrannosaurs might have:

Ripped through the neck or throat, causing teeth to meet vertebrae and the skull

Charged at prey with its mouth wide open to inflict a devastating wound, breaking teeth on whatever bones it hit

Attacked with a bite to the abdomen, hitting ribs in the process

Plus, an animal as large as T. rex may have needed bones for nourishment regardless of whether it killed an animal or found a corpse.

This counterargument even gets down to the defense mechanisms of T. rex's potential prey. Was the neck frill on a Triceratops for protection or sexual display? And did it use its horn for defense against predators or combat with intruders on its territory? Since we have no way to observe the behavior of these animals, we don't really know. The most logical answer may be that Tyrannosaurus rex, like many large predators, was an opportunist, catching fresh meat when possible and eating carrion when necessary.

To learn more about unsolved mysteries of the dinosaur world, see the links on the next page.

I'm Biting Your Head!

Regardless of whether Tyrannosaurus rex used its jaws to take down prey or bite through picked-over carcasses, it certainly used them for another purpose: biting its kin. Unearthed T. rex skull fossils display lots of evidence of bite wounds -- wounds that match up to the size, shape and position of tyrannosaur teeth. Whether this head biting was the result of mating rituals, territory defense or other social behaviors is unknown.